The present exemplary embodiments relate to phosphor compositions, particularly phosphors for use in lighting applications. More particularly, the present embodiments relate to red emitting phosphors and a lighting apparatus employing these phosphors.
Light emitting diodes (LEDs) are semiconductor light emitters often used as a replacement for other light sources, such as incandescent lamps. They are particularly useful as display lights, warning lights and indicator lights or in other applications where colored light is desired. The color of light produced by an LED is dependent on the type of semiconductor material used in its manufacture.
Colored semiconductor light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LEDs), have been produced from Group III-V alloys such as gallium nitride (GaN). With reference to the GaN-based LEDs, light is generally emitted in the UV to green range of the electromagnetic spectrum. Until recently, LEDs have not been suitable for lighting uses where a bright white light is needed, due to the inherent color of the light produced by the LED.
Recently, techniques have been developed for converting the light emitted from LEDs to useful light for illumination purposes. In one technique, the LED is coated or covered with a phosphor layer. A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range.
By interposing a phosphor excited by the radiation generated by the LED, light of a different wavelength, e.g., in the visible range of the spectrum, may be generated. Colored LEDs are often used in toys, indicator lights and other devices. Continuous performance improvements have enabled new applications for LEDs of saturated colors in traffic lights, exit signs, store signs, and the like.
In addition to colored LEDs, a combination of LED generated light and phosphor generated light may be used to produce white light. The most popular white LEDs consist of blue emitting GaInN chips. The blue emitting chips are coated with a phosphor that converts some of the blue radiation to a complementary color, e.g. a yellowish emission. Together, the blue and yellowish radiation produces a white light. There are also white LEDs that utilize a near UV emitting chip and a phosphor blend including red, green and blue emitting phosphors designed to convert the UV radiation to visible light.
Known light emitting devices comprise a blue light-emitting LED having a peak emission wavelength in the near blue range (from about 440 nm to about 480 nm) combined with a yellow light-emitting phosphor, such as cerium(III) doped yttrium aluminum garnet (“YAG:Ce”), a cerium(III) doped terbium aluminum garnet (“TAG:Ce”), or a europium(II) doped barium orthosilicate (“BOS:Eu”). The phosphor absorbs a portion of the radiation emitted from the LED and converts the absorbed radiation to a yellow light. The remainder of the blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. A viewer perceives the mixture of blue and yellow light as a white light. The total of the light from the phosphor material and the LED chip provides a color point with corresponding color coordinates (x and y) and correlated color temperature (CCT), and its spectral distribution provides a color rendering capability, measured by the color rendering index (CRI).
Such systems can be used to make light sources having CCTs of >4500 K and CRIs ranging from about 70-82, with luminous efficacy of radiation (“LER”, also referred to as luminosity) of about 330 lm/Wopt. While this range is suitable for many applications, general illumination sources usually require lower CCTs and higher CRIs, preferably with similar or better LER.
One particularly preferred application of LED devices made from blue emitting chips and yellow emitting phosphors is for backlights, e.g. in cellular phones, personal digital assistants, and the like. These applications require high CCT values (greater than 5000K), which are readily provided by the aforementioned LEDs, with LER values of 280 lm/Wopt or greater. However, due to the use of yellow phosphors (e.g. having a peak emission between 550 nm and 610 nm), the spectra of these LEDs contain excessive emission in the yellow region of the spectrum, which strongly reduces the color gamut of the backlight.
The color gamut is the area spanned between the color points of the red, green and blue pixels of the display in a chromaticity diagram, e.g. the CIE 1931 x, y diagram. The historical “golden standard” for displays is the NTSC gamut, defined by 3 sets of color point coordinates (x=0.674 and y=0.326 for red, x=0.218 and y=0.712 for green, and x=0.140 and y=0.080 for blue in the CIE 1931 x,y chromaticity diagram). Generally, a gamut greater than 70% of NTSC is considered acceptable for many backlighting applications, and a gamut greater than 90% of NTSC is considered acceptable for most any such application.
In order to improve the gamut of LED backlights using yellow phosphors, the yellow light is filtered out, thereby reducing the effective LER of the LED backlight. It would be therefore beneficial to develop LED backlights that can provide LER of 280 lm/Wopt or greater at CCT>5000K without having yellow phosphors in the package. Red line-emitting phosphors would be particularly suitable for obtaining simultaneously such values for LER and CCT when used with blue or near-UV LED chips, in conjunction with at least a green-emitting and a blue-emitting phosphor, as needed.
Other white light LED lighting systems use a UV or visible light LED chip along with a blend of red, green, and/or blue phosphors that can be efficiently excited by near-UV radiation to make white light.
The CRI is commonly defined as a mean value for 8 standard color samples (R1-8), usually referred to as the General Color Rendering Index and abbreviated as Ra, although 14 standard color samples are specified internationally and one can calculate a broader CRI (R1-14) as their mean value. In particular, the R9 value, measuring the color rendering for the strong red, is very important for a range of applications, especially of a medical nature.
As the CCT is lowered and/or the CRI is increased, the LER value for phosphor downconverted LEDs generally decreases, leading to values for “warm white” LEDs (of CCT<4500K) significantly lower than those for “cool white” LEDs (of CCT>4500K). The LER values can be improved by using red phosphors having a line emission spectrum, as opposed to red phosphors with a broadband emission. A significant portion of the emitted energy of the latter phosphors tends to fall into the deep red part of the spectrum where the sensitivity of the human eye is very low, thereby decreasing the LER value.
In many light applications, phosphors with line emission spectra (e.g. Y2O3:Eu3+) are thus preferred as the red component because it maximizes LER at acceptable CRI values (e.g. 80-86), over the CCT range of interest (e.g. 2700-6500K). Current red fluorescent lamp phosphors doped with Eu3+ such as Y2O3:Eu3+ has little absorption of near-UV (370-420 nm) light, leading to larger light losses due to scattering by phosphor powders. A deep red line emitting oxyfluoride phosphor, e.g. 3.5MgO*0.5MgF2*GeO2:Mn4+ (MFG) can be used in LED lamps, but with considerable impact to LER, due to much of its emission occurring too far in the deeper red region of the spectrum (beyond 650 nm). Other similar phosphors based on oxide and oxyhalide host lattices doped with Mn4+ also have a main emission peak at a wavelength greater than 650 nm. It would be preferable to use red phosphors with line emission maxima at lower wavelengths, e.g. in the 610 to 650 nm range, having good absorbance for the LED chip emitting in the UV to blue range. This improves the LER while maintaining the general CRI at 80 or above, as well as minimizes scattering losses within the LED package.
Thus, a continuing need exists for new red line-emitting phosphors and their blends for use in conjunction with UV and visible LED chips displaying high quantum efficiency to produce white-light LEDs having both a high CRI and an improved LER at any given CCT, as well as backlight LEDs with minimal yellow emission and high LER.